The present invention relates generally to motor control circuits. More particularly, the invention relates to a pulse width modulated DC motor speed control circuit with electronic clutch operable at a user-settable torque.
In power tool applications such as electric screwdrivers and nutdrivers it is desirable to provide a clutch mechanism by which the rotation of the tool is arrested once a predetermined torque is achieved. Ideally, this shutoff torque is user-settable, to allow the user to preselect how tightly the screw or nut should be driven. Conventionally, mechanical clutch mechanisms have been provided for this purpose. However, mechanical clutch mechanisms add considerably to the cost and weight of the tool. For this reason alternative means for electronically controlling the maximum torque output of a power tool have been proposed.
In particular, it is known that the torque output of a motor is directly related to the current drawn by the motor. Consequently, it has been proposed to control the torque output of a power tool by monitoring the current through the motor and interrupting the supply of power to the motor when the current reaches a preselected level. This approach to providing an electronic clutch function typically involves the application of a sensing resistor that is connected in series with the motor to sense the current flow through the motor. The signal provided by the sensing resistor is therefore proportional to motor current. However, because the current sensing resistor must carry the same level of current as the motor, a current sensing resistor typically comprises a low resistance, high wattage resistor which consequently consumes a substantial amount of power in the form of heat. For power tools operating off of AC power this is generally not a problem. However, with a cordless power tool operating off of a portable power source, the presence of such a current sensing resistor in the control circuit serves to reduce the useful operating time of the power tool between charges.
Consequently, it is desirable to provide a motor speed control circuit, and in particular one adapted for controlling a DC motor, that includes an electronic clutch function which minimizes the power dissipation associated with the sensing of motor current.
In addition, it is desirable to also use the sensed current signal to provide closed loop speed control of the motor as well.
The present invention accomplishes these objectives by providing a method and apparatus for controlling a motor to deliver a selected speed as well as a selected maximum torque. According to one aspect of the invention the method includes supplying drive current to the motor through a first port of an electronically controlled switching circuit and supplying sense current through a second port of the electronically controlled switching circuit. In the presently preferred form the switching circuit employs sense MOSFETs. A speed control signal is supplied to a control port of the switching circuit to cause the switching circuit to selectively change state, thereby controlling the speed of the motor. According to the method of the invention, a user-settable reference value is provided to indicate the desired shutoff torque. A peak value is derived from the controlled sense current and this value is compared with the user-settable reference value indicative of the desired shutoff torque. Based on the comparison, the motor is placed in an OFF state when the user-settable torque has been attained. According to another aspect of the invention, the motor is latched in the OFF state once the user-settable torque has been attained.
The apparatus of the invention comprises a motor control circuit for supplying energy from a power source to the motor in order to deliver a user-selected speed and a user-selected maximum torque. The control circuit comprises a switching means coupled to the power source which has a first port coupled to the motor for supplying drive current to the motor, a second port for supplying a sense signal proportional to the drive current and a third port or control port for causing the switching means to selectively change state, thereby controlling the speed of the motor. The circuit further includes a modulator means which is coupled to the control port for supplying a speed control signal to the control port for causing the switching means to change state. A user-settable means is provided to supply a torque signal indicative of the user-selected torque. The control circuit also includes a torque control comparator means coupled to the user-settable means and to the switching means for comparing the torque signal and the sense signal. The torque control comparator means also places the motor in an OFF state when the user selected torque has been attained.
For a more complete understanding of the invention, its objects and advantages, reference is made to the following specification and to the accompanying drawings.